1. Field of the Invention
Aspects of the present invention relate to a unit to remove crosstalk in a multi-layered disk, an optical pickup including the unit, and an optical recording and/or reproducing apparatus including the optical pickup.
2. Description of the Related Art
Optical disks, such as compact disks (CDs) and digital versatile disks (DVDs), are information storage media on and/or from which information is recorded and/or reproduced. Optical pickups record and/or reproduce information on and/or from an optical disk and include an objective lens that focuses a laser beam on the optical disk.
Much research has recently been conducted to develop next-generation high-density optical disks, such as high-definition digital versatile disks (HD-DVDs), Blu-ray disks (BDs), and advanced optical disks (AODs).
Such optical disks use laser light having different wavelengths and an objective lens having different numerical apertures (NA), depending on capacity of the optical disk. In optical recording and/or reproducing apparatuses that use a light spot on which a laser light is focused by an objective lens, in order to record and/or reproduce information on and/or from an information storage medium, the recording capacity is determined by the size S of the light spot. The size S of the light spot is determined by the wavelength λ of the laser light and the NA of the objective lens as shown in
                    S        ∝                              λ            NA                    .                                    (        1        )            
Accordingly, in order to increase the recording density of the optical disk, the wavelength of the laser light must be reduced and the NA of the objective lens must be increased.
However, components used to reduce the wavelength of the laser light are expensive. Also, an increase in the NA decreases the depth of focus in proportion to the square of the NA and increases coma aberration in proportion to the cube of the NA. Accordingly, there are limitations in increasing the recording density using the laser light and the objective lens.
Accordingly, in order to increase the recording density, a method of making a recording layer thicker has been proposed. However, in the case of a multi-layered disk, as a space between adjacent layers decreases, not only light reflected by a target recording layer but also light reflected by a recording layer adjacent to the target recording layer are detected by a photodetector and the light reflected by the adjacent recording layer (i.e., noise) affects a desired signal.
FIG. 1 illustrates light spots Lt and Ln formed on a 4-divided photodetector 3 of a conventional optical pickup. Referring to FIG. 1, light reflected by a target recording layer is focused into a light spot Lt on the photodetector 3. The light spot Lt is converted into an electrical signal which is output as a reproduction signal. For example, the reproduction signal may be detected as the sum signal of four light reception areas (A+B+C+D) of the photodetector 3. When the light reflected by the target recording layer from which information is to be reproduced is focused onto the light spot Lt on the photodetector 3, light reflected by a recording layer adjacent to the target recording layer reaches the photodetector 3 and forms a light spot Ln larger than the light spot Lt. Since the light spot Ln of the light reflected by the recording layer adjacent to the target recording layer is diffused and then formed all over the photodetector 3, the light spot Ln is combined with the light spot Lt from the target recording layer, thereby causing noise in the reproduction signal.